The present invention relates to a sodium cooled fast reactor for ensuring a reactor core coolant flow rate in a core emergency and improving a safeness of the core operation by a static system utilizing only a physical phenomenon in a reactor vessel.
Generally, in a fast reactor, an upper opening is closed by a roof slab and a core, in which a plurality of fuel assemblies are arranged, is disposed at substantially the central portion of the reactor vessel and supported by a core supporting structure disposed at the lower portion of the reactor vessel. A core upper structure is disposed at the upper portion of the core so as to penetrate the roof slab and a plurality of circulation pumps and intermediate heat exchangers are suspended from the roof slab at the outer peripheral portions of the core upper structure. A liquid metal sodium is utilized as a coolant for the fast reactor. The core is cooled by a sodium having a low temperature fed by means of the circulation pumps and the sodium heated after the cooling of the core is then cooled through the heat exchanging operation with a secondary sodium as a secondary coolant in the intermediate heat exchangers, the thus cooled sodium being thereafter again fed into the core by means of the corculation pumps.
One example of a conventional tank type fast reactor will be described hereunder with reference to FIG. 6.
Referring to FIG. 6, reference numeral 1 denotes a reactor vessel in which a liquid sodium 2 as a liquid metal coolant is accommodated. An upper opening of the reactor vessel 1 is closed by a roof slab 3. At the upper portion of the inside of the reactor vessel 1 there is formed a cover gas space 15 above the free surface level of the liquid sodium 2. A plurality of circulation pumps 4, only one being illustrated, are disposed so as to penetrate the roof slab 3 and a plurality of intermediate heat exchangers 6 only one being illustrated provided at its upper portions with secondary sodium inlet and outlet, are also disposed so as to penetrate the rool slab 3. The lower portions of the circulation pump 4 and the intermediate heat exchanger 6 penetrating downward through the roof slab 3. The intermediate heat exchanger 6 is provided with sodium inlet and outlet ports 5a and 5b positioned above the roof slab 3. A core 8 accommodating a plurality of fuel assemblies is mounted on a core support structure 9 at substantially the central portion of the reactor vessel 1.
In the tank type fast reactor of the structure described above, the coolant is forcibly circulated by means of the circulation pump and a low-temperature sodium 2a guided into a high-pressure plenum 17 is fed into the core 8 through an incore duct 16 to cool the core 8. A high-temperature sodium 2b after the cooling of the core 8 flows into a low-pressure plenum 11, upwardly radially along the lower portion of the core upper structure 10 as shown by arrows, and a portion of the high-temperature sodium 2b is guided into the intermediate heat exchanger 6 through windows 12 formed to the heat exchanger 6. The sodium 2b introduced into the heat exchanger 6 passes a plurality of electric heattubes, not shown, incorporated in the heat exchanger 6 to thereby carry out the heat exchanging operation with the secondary sodium, thus being cooled, and the sodium cooled after the heat exchanging operation then flows out into the high-pressure plenum 17 through an outlet nozzle 13 of the intermediate heat exchanger 6. The low-temperature sodium 2a flows out through the outlet nozzle 13 is guided to the circulation pump 4 and then introduced into the incore duct 16 and then to the core 8. The flows of the low- and high-temperature sodiums 2a and 2b are separated by two partition walls 7 composed of upper and lower partition wall sections.
In the conventional art, at an emergency core shutdown period of the sodium cooled fast reactor of the structure described above, control rods, not shown, are rapidly inserted into the core 8 to apply a negative reactivity to thereby stop a nuclear reaction, thus lowering the temperatures of the fuel and the liquid sodium 2. Simultaneously, the operation of the circulation pump 4 is stopped for alleviating cold shock to machinery disposed in the reactor vessel to thereby remove decay heat through a small flow rate operation by means of a pony motor.
In a case where the flow of the liquid sodium 2 is instantaneously stopped through the operation stop of the circulation pump 4, the flow rate of the liquid sodium for cooling the core 8 becomes short and the liquid sodium as the coolant may be boiled and, in an adverse case, the fuel may be damaged. In order to obviate such defect and to gently lower (coast down) the flow rate of the liquid sodium, in the conventional technology, when the circulation pump operation stops, a flow rate control device, not shown, for gently stopping the operation of the circulation pump 4 by a mechanical inertia due to a large-sized flywheel is installed at an external portion of the reactor.
However, a severe reliability is required for the flow rate control device in the view point of the safeness, and the flow rate control device is itself has a large structure for this purpose. Particularly, in a case where an electromagnetic pump is utilized for the circulation pump 4, the pump has no rotational inertia force, so that it becomes more important to install such flow rate control device with high operational reliability. Furthermore, in a case where the pump is stopped by, for example, an adhetion of a shaft of the pump or dielectric brakedown of the electromagnetic pump, there causes a case that the flow rate control device is not operated at all, thus providing a significant problem.